When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via a transvenous leadwire or catheter (referred to as a "lead") having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as "capturing" the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is the treatment of congestive heart failure (also referred to as "CHF"). Congestive heart failure, which can result from long-term hypertension, is a condition in which the walls of at least one side (e.g., the left side) of the heart become thin. As a result, the left atrium and left ventricle become disproportionately enlarged. The heart muscle associated with the left atrium and ventricle displays less contractility. This decreases cardiac output of blood through the circulatory system which, in turn, may result in an increased heart rate and less resting time between heartbeats. The heart consumes more energy and oxygen, and its condition typically worsens over a period of time.
As one side of the heart (e.g., the left side) becomes disproportionately enlarged, the intrinsic electrical heart signals that control heart rhythm are also affected. Normally, such intrinsic signals originate in the sinoatrial (SA) node in the upper right atrium, traveling through and depolarizing the atrial heart tissue such that resulting contractions of the right and left atria are triggered. The intrinsic atrial heart signals are received by the atrioventricular (AV) node which, in turn, triggers a subsequent ventricular intrinsic heart signal that travels through and depolarizes the ventricular heart tissue such that resulting contractions of the right and left ventricles are triggered substantially simultaneously.
Where one side (e.g., the left side) of the heart has become disproportionately enlarged due to congestive heart failure, however, the ventricular intrinsic heart signals may travel through and depolarize the left side of the heart more slowly than in the right side of the heart. As a result, the left and right ventricles do not contract simultaneously, but rather, the left ventricle contracts after the right ventricle. This delay between right ventricular and left ventricular contractions reduces the pumping efficiency of the heart due to movement of the septal wall between right and left sides of the heart. Congestive heart failure may also result in an another symptom, that is, an overly long delay between atrial and ventricular contractions. This too-long delay between atrial and ventricular contractions also reduces the pumping efficiency of the heart. There is a need to provide congestive heart failure patients with therapy that improves heart pumping efficiency.
Conventional cardiac rhythm management techniques, however, are typically directed toward treating the right side of the heart, which pumps blood to the lungs. For example, endocardial leads are typically designed to be inserted via the superior vena cava into one or more of the right atrium and right ventricle. Because the left side of the heart pumps blood throughout the patient's peripheral circulatory system, pressures are typically higher in the left side of the heart than on the right side of the heart. Because access to the left side of the heart is more difficult, and because a thrombus forming on a left side lead could cause a stroke or a myocardial infarction, it is typically very difficult to chronically implant an endocardial catheter leads directly into the left atrium and left ventricle of the heart.
Another problem with treating congestive heart failure patients involves sensing intrinsic heart signals. Cardiac rhythm management devices typically sense intrinsic atrial and ventricular heart signals, and adjust the therapy being delivered to the heart based at least in part on events detected from these sensed signals or from the delivery of the therapy itself. Such events are also referred to as "beats," "activations," "depolarizations," or "contractions," and are sensed via one or more electrodes located at or near that portion of the heart from which the sensed signals are to be obtained. Atrial depolarizations are also referred to as "P-waves." Ventricular depolarizations are also referred to as "QRS complexes," or "R-waves." Congestive heart failure, however, may result in a significant delay between right and left ventricular contractions, as discussed above. Such delays not only decrease the pumping efficiency of the heart, they may also result in the sensing of a right ventricular depolarization that is separated in time from a sensed left ventricular depolarization.
In order to properly deliver therapy to the heart based on sensed events, the cardiac rhythm management system must be able to distinguish between sensed right and left ventricular depolarizations that are separated in time because of delayed conduction through an enlarged left ventricle, and successive depolarizations originating in the same heart chamber that represent successive contractions of the same heart chamber. For example, if the cardiac rhythm management system mistakenly recognizes a right ventricular depolarization followed shortly by a left ventricular depolarization as a pair of successive right ventricular depolarizations, then therapy (such as, for example, a defibrillation countershock) may be delivered inappropriately, particularly if this behavior is sensed repeatedly over several cardiac cycles. Because defibrillation countershocks are typically quite painful to the patient and may further irritate the heart, the inappropriate delivery of defibrillation countershocks should be avoided, if possible. Similarly, therapy (such as, for example, a pacing stimulus) may be inappropriately withheld (i.e., "inhibited") because the left ventricular depolarization is mistaken for a subsequent right ventricular depolarization. There is a need for improved sensing and event recognition techniques that reduce the likelihood that left ventricular depolarizations are mistakenly recognized as right ventricular depolarizations (or vice-versa) so that the cardiac rhythm management system can provide appropriate therapy to the patient based on sensed events.